This application claims the benefit of Japanese Application No. 2001-187532 filed Jun. 21, 2001.
The present invention relates to an external magnetic field measuring method, a static magnetic field correcting method, an external magnetic field measuring apparatus, and a magnetic resonance imaging (MRI) system. More particularly, the present invention relates to an external magnetic field measuring method for measuring the strength of a magnetic field applied externally to an MRI system. The present invention also relates to a static magnetic field correcting method for correcting a variation in the strength of a static magnetic field caused by a magnetic field applied externally to an MRI system. Moreover, the present invention relates to an external magnetic field measuring apparatus for measuring the strength of a magnetic field applied externally to an MRI system. Furthermore, the present invention relates to an MRI system having the ability to correct a variation in the strength of a static magnetic field caused by a magnetic field applied externally thereto.
In MRI systems, a magnet assembly generates a static magnetic field of a predetermined strength. However, the strength of the static magnetic field is varied with application of an external magnetic field to an MRI system.
In efforts to prevent the variation in the strength of a static magnetic field caused by an external magnetic field, the magnet assembly included in the MRI system may be stored in a shield room or buried under the ground (Japanese Unexamined Patent Publication No. 2000-70245).
However, when only the passive method of storing a magnet assembly of an MRI system in a shield room or burying the magnet assembly under the ground is adopted, a variation in the strength of a static magnetic field caused by an external magnetic field cannot be prevented satisfactorily.
The first object of the present invention is to provide an external magnetic field measuring method and apparatus for measuring the strength of a magnetic field applied externally to an MRI system.
The second object of the present invention is to provide a static magnetic field correcting method for correcting a variation in the strength of a static magnetic field caused by a magnetic field externally applied to an MRI system, and to provide an MRI system having the ability to correct the variation in the strength of a static magnetic field caused by a magnetic field applied externally thereto.
From the first aspect of the present invention, there is provided an external magnetic field measuring method according to which a magnetization detecting means is placed near a magnet assembly included in an MRI system. A magnetic field generating means is located near the magnetization detecting means. The magnetic field generating means generates a compensation magnetic field so that the compensation magnetic field will cancel a magnetic field generated by the magnet assembly and detected by the magnetization detecting means in the absence of an external magnetic field. The magnetization detecting means measures the external magnetic field with the compensation magnetic field generated.
In order to highly precisely measure an external magnetic field that adversely affects a static magnetic field generated in an MRI system, a magnetization detecting means whose dynamic range permits high-precision detection of an external magnetic field is preferably located near a magnet assembly. This is however unfeasible because near the magnet assembly, a static magnetic field generated by the magnet assembly is much stronger than the external magnetic field. Therefore, if the magnetization detecting means whose dynamic range permits high-precision detection of an external magnetic field is adopted, the strength of the static magnetic field generated by the magnet assembly exceeds the dynamic range. This results in a failure to measure the external magnetic field. In reality, therefore, the dynamic range of the magnetization detecting means is expanded or the magnetization detecting means is located apart from the magnet assembly. If the former countermeasure is adopted, the external magnetic field cannot be detected high precisely. In contrast, if the latter is adopted, it is uncertain whether an external magnetic field being measured is an external magnetic field that adversely affects a static magnetic field generated in the MRI system.
In contrast, according to the external magnetic field measuring method provided from the first aspect, the magnetic field generating means generates a compensation magnetic field so that the compensation magnetic field will cancel a magnetic field generated by the magnet assembly. The magnetization detecting means whose dynamic range permits high-precision detection of an external magnetic field can therefore be located near the magnet assembly. Consequently, an external magnetic field that adversely affects a static magnetic field generated in an MRI system can be measured highly precisely.
From the second aspect of the present invention, there is provided an external magnetic field measuring method based on the aforesaid external magnetic field measuring method. Herein, when the magnetization detecting means includes a Z-direction magnetic sensor. Assuming that the direction of a static magnetic field generated by the magnet assembly is regarded as a Z direction, the Z-direction magnetic sensor detects a magnetization exhibited in the Z direction.
According to the external magnetic field measuring method provided from the second aspect of the present invention, the Z-direction magnetic sensor is included. Therefore, the major component of an external magnetic field that is directed in the same direction as the static magnetic field and that adversely affects the static magnetic field generated in an MRI system can be measured preferably.
From the third aspect of the present invention, there is provided an external magnetic field measuring method based on the aforesaid external magnetic field measuring methods. Herein, the magnetization detecting means includes a Z-direction magnetic sensor, a Y-direction magnetic sensor, and an X-direction magnetic sensor. Assuming that the direction of a static magnetic field generated by the magnet assembly is regarded as a Z direction and that two mutually orthogonal directions orthogonal to the Z direction are regarded as Y and X directions respectively, the Z-direction magnetic sensor, Y-direction magnetic sensor, and X-direction magnetic sensor detect magnetizations exhibited due to the magnet in the Z, Y, and X directions respectively.
According to the external magnetic field measuring method provided from the third aspect of the present invention, the Z-direction magnetic sensor is included. Therefore, the major component of an external magnetic field that adversely affects the static magnetic field generated in an MRI system and that is directed in the same direction as the static magnetic field can be measured preferably. Moreover, since the Y-direction magnetic sensor and X-direction magnetic sensor are also included, the components of the external magnetic field other than the major component that adversely affect the static magnetic field generated in the MRI system can be measured preferably.
From the fourth aspect of the present invention, there is provided an external magnetic field measuring method based on the aforesaid external magnetic field measuring methods. Herein, the magnetic field generating means includes at least a pair of small-size coils and a coil drive circuit. The pair of small-size coils is arranged to sandwich the Z-direction magnetic sensor in the Z direction. The coil drive circuit feeds a current to the pair of small-size coils.
According to the external magnetic field measuring method provided from the fourth aspect of the present invention, a current flows into the pair of small-size coils that is opposed to each other in the Z direction. Consequently, a compensation magnetic field can be preferably generated for canceling a static magnetic field that is generated in an MRI system and applied to the Z-direction magnetic sensor.
From the fifth aspect of the present invention, there is provided an external magnetic field measuring method based on the aforesaid external magnetic field measuring methods. Herein, the magnetic field generating means includes three pairs of small-size coils and a coil drive circuit. One of the three pairs of small-size coils is arranged to sandwich the Z-direction magnetic sensor in the Z direction. Other pair of small-size coils is arranged to sandwich the Y-direction magnetic sensor in the Y direction. The other pair of small-size coils is arranged to sandwich the X-direction magnetic sensor in the X direction. The coil drive circuit feeds a current to the small-size coils.
According to the external magnetic field measuring method provided from the fifth aspect of the present invention, a current flows into the pair of small-size coils that is opposed to each other in the Z direction. Consequently, a compensation magnetic field is preferably generated for canceling the major component, that is, the Z-direction component of a static magnetic field that is generated in an MRI system and that is applied to the Z-direction magnetic sensor. Moreover, a current flows into the pair of small-size coils that is opposed to each other in the Y direction. Consequently, a compensation magnetic field is preferably generated for canceling the Y-direction component of the static magnetic field that is generated in the MRI system and that is applied to the Y-direction magnetic sensor. Moreover, a current flows into the pair of small-size coils that is opposed to each other in the X direction. Consequently, a compensation magnetic field is preferably generated for canceling the X-direction component of the static magnetic field that is generated in the MRI system and that is applied to the X-direction magnetic sensor.
From the sixth aspect of the present invention, there is provided an external magnetic field measuring method based on the aforesaid external magnetic field measuring methods. Herein, the magnetization detecting means is placed above the magnet assembly.
Among the spaces present near the magnet assembly included in an MRI system, the spaces present back and forth and right and left are occupied with a table, a control unit, an operation unit, and others. Placement of the magnetization detecting means in the space present below the magnet assembly makes it hard to maintain the magnetization detecting means.
According to the external magnetic field measuring method provided from the sixth aspect of the present invention, the magnetization detecting means is placed in the space present above the magnet assembly. Therefore, the magnetization detecting means will not interfere with placement of the table, control unit, and operation unit, but can be maintained easily.
From the seventh aspect of the present invention, there is provided an external magnetic field measuring method based on the aforesaid external magnetic field measuring methods. Herein, the compensation magnetic field is corrected based on temperature.
In particular, when the magnet assembly includes a permanent magnet, the strength of a static magnetic field is liable to vary with a change in temperature.
According to the external magnetic field measuring method provided from the sixth aspect of the present invention, the compensation magnetic field is corrected based on temperature. Therefore, despite a change in temperature, a magnetic field generated by the magnet assembly can be canceled preferably.
From the eighth aspect of the present invention, there is provided a static magnetic field correcting method. Herein, a magnetic field correction coil is added to a yoke included in a magnet assembly of an MRI system. A correction current that is proportional to an external magnetic field measured according to the external magnetic field measuring method provided from any of the first to seventh aspects of the present invention is fed to the magnetic field correction coil. A correction magnetic field is thus generated in order to correct a static magnetic field.
According to the static magnetic field correcting method provided from the eighth aspect of the present invention, a correction current that is proportional to an external magnetic field detected highly precisely is fed to the correction coil. A correction magnetic field is thus applied to the yoke included in the magnet assembly, whereby the adverse effect of the external magnetic field on the static magnetic field can be nullified highly precisely.
From the ninth aspect of the present invention, there is provided a static magnetic field correcting method. Herein, assuming that the direction of a static magnetic field generated by a magnet assembly included in an MRI system is regarded as a Z direction, a pair of large-size coils is arranged to sandwich the magnet assembly in the Z direction. A correction current that is proportional to an external magnetic field measured according to the external magnetic field measuring method provided from any of the first to seventh aspects of the present invention is fed to the large-size coils. Correction magnetic fields are thus generated in order to correct the static magnetic field.
According to the static magnetic field correcting method provided from the ninth aspect of the present invention, a correction current that is proportional to an external magnetic field detected highly precisely is fed to the large-size coils that are opposed to each other in the Z direction. Correction magnetic fields that are directed in the Z direction are thus applied, whereby the Z-direction component, that is, the major component of the external magnetic field that adversely affects the static magnetic field can be canceled highly precisely.
From the tenth aspect of the present invention, there is provided a static magnetic field correcting method. Herein, assuming that the direction of a static magnetic field generated by a magnet assembly included in an MRI system is regarded as a Z direction and that two mutually orthogonal directions orthogonal to the Z direction are regarded as Y and X directions respectively, a pair of large-size coils is arranged to sandwich the magnet assembly in the Z direction. Another pair of large-size coils is arranged to sandwich the magnet assembly in the Y direction, and another pair of large-size coils is arranged to sandwich the magnet assembly in the X direction. A correction current that is proportional to an external magnetic field measured according to any of the aforesaid external magnetic field measuring methods is fed to the large-size coils. Thus, correction magnetic fields are generated in order to correct the static magnetic field.
According to the static magnetic field correcting method provided from the tenth aspect of the present invention, a correction current that is proportional to an external magnetic field detected highly precisely is fed to the large-size coils that are opposed to each other in the Z direction. Correction magnetic fields that are directed in the Z direction are thus applied. Consequently, the Z-direction component, that is, the major component of the external magnetic field that affects the static magnetic field can be canceled highly precisely. Moreover, the correction current proportional to the external magnetic field detected highly precisely is fed to the large-size coils, which are opposed to each other in the Y direction, in order to apply correction magnetic fields that are directed in the Y direction. Consequently, the Y-direction component of the external magnetic field that affects the static magnetic field can be canceled highly precisely. Moreover, the correction current proportional to the external magnetic field detected highly precisely is fed to the large-size coils, which are opposed to each other in the X direction, in order to apply correction magnetic fields that are directed in the X direction. Consequently, the X-direction component of the external magnetic field that affects the static magnetic field can be canceled highly precisely.
From the eleventh aspect of the present invention, there is provided an external magnetic field measuring apparatus. The external magnetic field measuring apparatus includes a magnetization detecting means and a magnetic field generating means. The magnetization detecting means is placed near a magnet assembly included in an MRI system. The magnetic field generating means is located near the magnetization detecting means. The magnetic field generating means generates a compensation magnetic field so that the compensation magnetic field will cancel a magnetic field that is generated by the magnet assembly and detected by the magnetization detecting means in the absence of an external magnetic field.
In the external magnetic field measuring apparatus provided from the eleventh aspect of the present invention, the external magnetic field measuring method provided from the first aspect of the present invention can be implemented preferably.
From the twelfth aspect of the present invention, there is provided an external magnetic field measuring apparatus based on the aforesaid external magnetic field measuring apparatus. Herein, the magnetization detecting means includes a Z-direction magnetic sensor. Assuming that the direction of a static magnetic field generated by the magnet assembly is regarded as a Z direction, the Z-direction magnetic sensor detects a magnetization exhibited in the Z direction.
In the external magnetic field measuring apparatus provided from the twelfth aspect of the present invention, the external magnetic field measuring method provided from the second aspect thereof can be implemented preferably.
From the thirteenth aspect of the present invention, there is provided an external magnetic field measuring apparatus based on the aforesaid external magnetic field measuring apparatuses. Herein, the magnetization detecting means includes a Z-direction magnetic sensor, a Y-direction magnetic sensor, and an X-direction magnetic sensor respectively. Assuming that the direction of a static magnetic field generated by the magnet assembly is regarded as a Z direction and that two mutually orthogonal directions orthogonal to the Z direction are regarded as Y and X directions respectively, the Z-direction magnetic sensor, Y-direction magnetic sensor, and X-direction magnetic sensor detect magnetizations exhibited due to the magnet assembly in the Z, Y, and X directions respectively.
In the external magnetic field measuring apparatus provided from the thirteenth aspect of the present invention, the external magnetic field measuring method provided from the third aspect thereof can be implemented preferably.
From the fourteenth aspect of the present invention, there is provided an external magnetic field measuring apparatus based on the aforesaid external magnetic field measuring apparatuses. Herein, the magnetic field generating means includes at least a pair of small-size coils and a coil drive circuit. The pair of small-size coils is arranged to sandwich the Z-direction magnetic sensor in the Z direction. The coil drive circuit feeds a current to the small-size coils.
In the external magnetic field measuring apparatus provided from the fourteenth aspect of the present invention, the external magnetic field measuring method provided from the fourth aspect thereof can be implemented preferably.
From the fifteenth aspect of the present invention, there is provided an external magnetic field measuring apparatus based on the aforesaid external magnetic field measuring apparatuses. Herein, the magnetic field generating means includes three pairs of small-size coils and a coil drive circuit. One of the three pairs of small-size coils is arranged to sandwich the Z-direction magnetic sensor in the Z direction. Other pair of small-size coils is arranged to sandwich the Y-direction magnetic sensor in the Y direction. The other pair of small-size coils is arranged to sandwich the X-direction magnetic sensor. The coil drive circuit feeds a current to the small-size coils.
In the external magnetic field measuring apparatus provided from the fifteenth aspect of the present invention, the external magnetic field measuring method provided from the fifth aspect thereof can be implemented preferably.
From the sixteenth aspect of the present invention, there is provided an external magnetic field measuring apparatus based on the aforesaid external magnetic field measuring apparatuses. Herein, the magnetization detecting means is placed above the magnet assembly.
In the external magnetic field measuring apparatus provided from the sixteenth aspect of the present invention, the external magnetic field measuring method provided from the sixth aspect thereof can be implemented preferably.
From the seventeenth aspect of the present invention, there is provided an external magnetic field measuring apparatus based on the aforesaid external magnetic field measuring apparatuses. Herein, the external magnetic field measuring apparatus includes a temperature correcting means for correcting the compensation magnetic field according to temperature.
In the external magnetic field measuring apparatus provided from the seventeenth aspect of the present invention, the external magnetic field measuring method provided from the seventh aspect thereof can be implemented preferably.
From the eighteenth aspect of the present invention, there is provided an MRI system including a magnet assembly, a magnetic field correction coil, and a magnetic field correction coil power supply. The magnet assembly includes a yoke. The magnetic field correction coil is added to the yoke and generates a correction magnetic field. The magnetic field correction coil power supply feeds a correction current, which is proportional to an external magnetic field measured by the external magnetic field measuring apparatus provided from any of the eleventh to seventeenth aspects of the present invention, to the magnetic field correction coil. Consequently, a correction magnetic field is generated in order to correct a static magnetic field.
In the MRI system provided from the eighteenth aspect of the present invention, the static magnetic field correcting method provided from the eighth aspect thereof can be implemented preferably.
From the nineteenth aspect of the present invention, there is provided an MRI system including a pair of large-size coils and a correction current feeding power supply. Assuming that the direction of a static magnetic field generated by a magnet assembly included in an MRI system is regarded as a Z direction, the pair of large-size coils is arranged to sandwich the magnet assembly in the Z direction. The correction current feeding power supply feeds a correction current, which is proportional to an external magnetic field measured by any of the aforesaid external magnetic field measuring apparatuses, to the large-size coils. Correction magnetic fields are thus generated in order to correct the static magnetic field.
In the MRI system provided from the nineteenth aspect of the present invention, the static magnetic field correcting method provided from the ninth aspect thereof can be implemented preferably.
From the twentieth aspect of the present invention, there is provided an MRI system including three pairs of large-size coils and a correction current power supply. Assuming that the direction of a static magnetic field generated by a magnet assembly included in the MRI system is regarded as a Z direction and that two orthogonal directions orthogonal to the Z direction are regarded as Y and X directions respectively, one of the three pairs of large-size coils is arranged to sandwich the magnet assembly in the Z direction. Other pair of large-size coils is arranged to sandwich the magnet assembly in the Y direction. The other pair of large-size coils is arranged to sandwich the magnet assembly in the X direction. The correction current power supply feeds a correction current, which is proportional to an external magnetic field measured by any of the aforesaid external magnetic field measuring apparatuses, to the large-size coils. Correction magnetic fields are thus generated in order to correct the static magnetic field.
In the MRI system provided from the twentieth aspect of the present invention, the static magnetic field correcting method provided from the tenth aspect thereof can be implemented preferably.
According to an external magnetic field measuring method and an external magnetic field measuring apparatus in which the present invention is implemented, a magnetization detecting means whose dynamic range permits high-precision detection of an external magnetic field can be placed near a magnet assembly. Consequently, an external magnetic field that adversely affects a static magnetic field generated in an MRI system can be measured highly precisely.
Moreover, according to a static magnetic field correcting method and an MRI system in which the present invention is implemented, the adverse effect of an external magnetic field on the static magnetic field can be canceled because the external magnetic field can be detected highly precisely.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.